Unit 2 Lectures 2, 3 & 4 - The Lipid Bilayer

Features of biological membranes

1. Bilayer of phospholipids

2. Organized and fluid

3. Different permeability for different types of molecules

4. Asymmetric

Role of lipids in the membrane

- impermeable barrier to water-soluble materials

- electrical resistance

- flexibility

- stability

Types of Lipids

1. Phospholipids

2. Sterols

3. Fatty Acids

4. Triacylglyerols

Why do phospholipids form a bilayer?

Hydrophilic heads attracted by water so are on the outside

Hydrophobic tails repel water so point inwards

What form do fatty acids take in aqueous environments?

micelle

What forms do phospholipids take in aqueous environments?

bilayer and liposome

What form do sterols take in aqueous environments?

monolayer on their own with the hydroxyl group interacting with water

they can be inserted into the phospholipid bilayer

What form do triacylglycerides take in aqueous environments?

oil droplets

Requirements for bilayer formation

1. Amphipathicity

2. Correct shape (head + 2 tails makes cylinder shape)

Thermodynamics of the hydrophobic effect (minimum energy conformation)

the minimum energy conformation is achieved by minimizing the exposure of hydrophobic groups to water

Thermodynamics of the hydrophobic effect (free energy)

the free energy of the system is minimized if the hydrophobic regions cluster together to limit contact with water

Thermodynamics of the hydrophobic effect (motional freedom)

water achieves the greatest possible motional freedom if its contact is limited with hydrophobic groups

Why is there no water within the hydrophobic core of the bilayer

water cannot hydrogen bond with the fatty acid tail, causing it to be shielded from the fatty acid core by the polar heads

Biological membrane makeup

phospholipids, glycolipids, sterols

Lateral diffusion within the fluid mosaic model

phospholipids can diffuse laterally because, in doing so, they remain at the same energy level and are under the same conditions

Transverse diffusion within the fluid mosaic model

It requires a lot of energy for polar head groups to move through the hydrophobic section of the lipid bilayer; flippases make this possible

FRAP

Fluorescent Recovery After Photobleaching

What does FRAP measure?

rate of diffusion of membrane proteins (lateral recovery)

How does FRAP work?

1. fluorescent marker is bound to the phospholipids that make up a cell membrane.

2. laser is used to bleach a small patch of membrane (stop the patch from fluorescing)

3. patch will again show fluorescence due to lateral diffusion of phospholipids throughout the membrane OR no fluorescence will be shown

What does it mean when a patch recovers quickly from bleaching?

the membrane is more fluid/mobile; it's proteins are not anchored

What does it mean when a patch recovers slowly from bleaching?

the membrane is less fluid/mobile; it's proteins are anchored

Effect of temperature on lipids

at cold temperatures, membrane bilayers freeze and assume a crystalline state with low fluidity and high fragility

[Plants and fungi/Animals] use tactics to maintain fluidity more often?

Plants and fungi; they are less capable of moving to a warmer space

Tactics to increase fluidity

- increase number of unsaturated lipids

- decrease tail length

- increase amount of sterols

How does increasing the number of unsaturated lipids maintain fluidity?

a higher number of saturated lipids means that the bilayer is more tightly packed with more van der Waals interactions and is less fluid; the fluidity can be increased by increasing the amount of unsaturated fatty acids since there are more kinks in the tails

How does decreasing tail length maintain fluidity?

shorter tails are more fluid since they have less surface area to form van der Waals interactions

How does changing the amount of sterols maintain fluidity?

- at low temperatures, sterols decrease van der Waals and increase fluidity

- at high temperatures, sterols introduce more surface area and more van der Waals interactions, leaving the membrane less fluid

Lipid Rafts

lipid-rich microdomains that are more ordered than the supporting membrane

Methods of membrane adhesion to other molecules

- adhesion to proteins inside the cell

- adhesion to extracellular matrix

- adhesion to neighbouring cells

- adhesions to proteins inside the cell (in the cell cortex)

The plasma membrane is reinforced inside the cell by

association of membrane proteins with the cell cortex

Adhesion to extracellular matrix

integrin proteins can connect to the extracellular matrix

Adhesion to neighbouring cells

cell to cell adhesion molecules (cadherins) linking the plasma membranes of neuronal cells

Tight junctions

- adhesions between neighbouring epithelial cells so nothing leaks between the cells

- segregates apical and basolateral sides of cells into distinct membrane domains

- especially important for cells in the intestinal epithelium (leaky gut)

Faulty tight junctions

permits undigested food particles, microorganisms, and toxins to pass through the epithelium into capillaries (leaky gut)

Anchored proteins with FRAP

after photobleaching, the area of a protein with anchored proteins will remain bleached. this is because the protein is anchored and cannot undergo lateral diffusion.

Unanchored proteins with FRAP

after photobleaching, the area of a protein with unanchored proteins will regain brightness. this indicates that the membrane is fluid and proteins are able to disperse via lateral diffusion.

Permeability of the lipid bilayer for gases and hydrophobic molecules

diffuse freely

Permeability of the lipid bilayer for small uncharged polar molecules

diffuse fairly well

Permeability of the lipid bilayer for large, uncharged, polar molecules

diffusion is negligible

Permeability of the lipid bilayer for ions

diffusion is not possible

How do proteins change membrane permeability?

proteins allow the cell to be selectively permeable; each organelle has its own subset of unique channels and carriers

N terminus

the end of a polypeptide or protein that has a free amino group

C terminus

the end of a polypeptide or protein that has a free carboxyl group

Amino acid residue

an amino acid that is part of a peptide, polypeptide, or protein chain

Bonding/interactions involved in primary structure

covalent (peptide) bonds between amino acids

Bonding/interactions involved in secondary structure

backbone interactions via hydrogen bonding

Alpha helix

- repetitive hydrogen bonds all the way along and parallel to the backbone of the helix

- r groups project outward

Beta pleated sheets

- can be parallel or antiparallel

- r groups project upward or downward away from the peptide backbone

Sides of the beta pleated sheet are [identical/distinct]

distinct

Secondary structure elements fold into ___________ within a tertiary structure

domains

Different domains are associated with different functions, including

- transmembrane domain

- DNA binding domain

- catalytic domain

Bonding/interactions involved in tertiary and quaternary structure

- hydrogen bonds

- ionic bonds

- van der Waals interactions

- disulfide bonds

Quaternary structure stabilization

- stabilized by all factors that stabilize the tertiary structure

- the 3D configuration of different polypeptides that make up a molecular complex consisting of several subunits

- stabilized by a combination of hydrophilic or hydrophobic interactions (or both) between polypeptides

Amino acid side chains on the side of a final, folded protein complex

hydrophilic

Protein structure formation thermodynamics

- increase the stability of the system by having non-covalent and covalent interactions

- allow for molecular self-assembly

Primary structure

linear amino acid sequence of peptide-bonded amino acids; determines the protein's 3D structure

Secondary structure

local 3-D structure stabilized by backbone H-bonding of the peptide; examples include alpha-helices and beta-sheets

Tertiary Structure

overall 3-D structure ('fold') of entirepolypeptide; stabilized by side-chain interactions (non-covalent anddisulfide bonds) as well as interactions between side chains andbackbone atoms

Quaternary structure

3-D arrangement of polypeptides in aprotein composed of multiple subunits; similar stabilization astertiary

Integral membrane proteins

Proteins directly attached to the membrane ;amphipathic; can be monomeric or multimeric

Peripheral membrane proteins

Bound to membrane surfaces through non-covalent association with other membrane proteins

Asymmetry with integral membrane proteins

the orientation of transmembrane proteins matter; the leaflet of attachment matters

Asymmetry with peripheral membrane proteins

different proteins attach to different sides

Hydropathy plots

plots measuring the hydrophobicity of proteins; peaks above the threshold indicate potential transmembrane alpha helices; used to predict the number and orientation of alpha helical transmembrane segments

Characteristics of a membrane spanning domain?

non-polar section for the interior of the membrane to interact with phospholipid tails; polar section for the exterior of the membrane to interact with the polar heads

Peripheral protein attachment to membrane

attached indirectly through non-covalent interactions

Isolating peripheral proteins

use high salt to weaken protein-protein interactions by disrupting electrostatic bonds

Integral protein attachment to membrane

van der Waals interactions

Isolating integral membrane proteins

use detergent (SDS, Triton X-100) to solubilize the proteins

Protein electrophoresis gel

polyacrylamide

DNA electrophoresis gel

agarose

Protein electrophoresis

- different charges and shapes because of amino acid sequence

- a strong ionic detergent helps equalize the charge-to-mass ratio and also denatures proteins so that they separate according to size

DNA electrophoresis

- equal charge-to-mass ratio throughout the length of each molecule

- separates according to size

Membrane asymmetry

different lipids in the extracellular side and the cytosolic side

Flippases

specific membrane proteins that maintain the bidirectional transport of lipids between the layers of the phospholipid bilayer in cells

Floppases

move amphiphilic lipids from inner leaflet to outer leaflet

ATP dependent

Scramblases

remove randomly selected phospholipids from one half of the lipid bilayer and insert them in the other